Air-conditioning manufacturers, architects, and professional design engineers have expended huge efforts in optimizing the design of building air-conditioning systems. Annual sales of equipment amount to tens of billions of dollars and annual energy use for heating and cooling have similar values. In addition the costs associated with reduced productivity of workers because of uncomfortable environmental conditions may be several times these figures, although difficult to quantify. Yet despite this effort the fundamental process for air conditioning buildings has remained essentially the same since the introduction of the first air conditioners in the 1920""s. Conventional approaches to air conditioning have inherent problems that severely limit their efficiency, raise installed cost, and frequently produce poor comfort conditions in the building space. Solving these problems requires major changes in the basic configuration of air-conditioning systems.
Conventional air-conditioning systems use a relatively small volume of air for cooling. The typical arrangement uses a vapor-compression refrigeration system to cool a mixture of return air and outside air to approximately 55xc2x0 F. and then distribute the cooled air through ducts to the building space. The low supply air temperatures are a result of the need to cool air below its dew point to remove moisture. The low air temperatures are also necessary to meet the sensible cooling needs of the space without excessively large ducts.
There are several important problems with this approach. The first is related to fan energy use. Since air flow is through relatively restrictive ductwork, fan static pressures are quite high. Typical pressures range from less than 0.5 inches of water for residential systems to as much as 5 to 10 inches of water for large commercial cooling systems. These high static pressures result in large energy use from the fan, which also adds to the cooling load for the rest of the system. In many commercial systems, the fan heat accounts for as much as 20 to 30 percent of the total cooling load for the building. The net result is a very inefficient cooling system.
A second problem is with high compressor energy required. The low supply air temperatures mean even lower evaporating temperatures, typically 40 to 50 F for the compressor system. The low evaporating temperatures create more work for the compressor, which further reduces the efficiency of the system.
A third problem is poor indoor air quality associated with high duct humidity. Conditions over 70% relative humidity allow the growth of mold and fungus in ductwork. The relative humidity in the supply ducts for conventional systems is frequently over 90%. In addition water from wet coils wets drain pans and can also wet nearby ductwork. These wet conditions create a potential breeding grounds for many types of microbes that can cause health and odor problems.
A fourth issue is high noise levels with conventional systems. The high static pressure creates a need for a powerful fan that usually is quite noisy. In addition, metal ducts transmit the noise quite well. Common fixes for the noise problem include the use of fiberglass duct liners. Unfortunately these liners increase cost and pressure drop and also can contribute to problems with molds given the high relative humidity in most ducts.
A fifth problem is the potential for drafts with conventional cooling systems. The low supply air temperatures and high velocities create the possibility of extremely uncomfortable conditions near the vents. Designers must take special care to ensure adequate mixing of room air and supply air to reduce drafts to acceptable levels.
A sixth problem is the need for simultaneous heating and cooling. Most office buildings have a single air handling system for the interior and exterior zones. In cold weather the interior zones still need cooling because of heat from people, lights, equipment, etc., while the exterior needs heat. The usual solution is to supply cool air to the entire building in order to satisfy the cooling needs of the interior. Perimeter heaters or heaters in the ducts servicing the exterior zones then provide the heat necessary to satisfy the heating load and overcome the cooling from the supply air.
The objective of the present invention is to improve energy efficiency and to reduce or eliminate the problems associated with existing air conditioning systems.
The invention uses a fundamentally different approach to air conditioning. The approach involves the use of a large volumetric flow rate of air with a temperature that is close to that of the building space for space heating and cooling. A separate dehumidification system is used in humid climates. In one preferred embodiment, a ceiling plenum is used as for the supply air and air returns through the space. In another preferred embodiment, supply air enters the space through a vent near the ceiling along one wall and returns near the floor along the same wall. Pressure drops are kept very low because of the low air velocities. The low pressure and small temperature difference between the supply air and the room air allow for very low energy use and improved comfort.